1. Field of the Invention
The present invention relates to new and improved reactor core designs for fast reactors, particularly liquid metal or molten salt cooled nuclear reactors.
2. Description of the Prior Art
As described in Westinghouse U.S. Pat. No. 4,949,363 (Tupper et al.), a liquid metal-cooled nuclear reactor (LMR), like other nuclear reactors, operate at temperatures up to 538 C.° (1,000° F.), and produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear reactor core situated in a reactor vessel. The heat produced by the LMR is used to generate electricity.
Liquid sodium has excellent heat-transfer properties and low vapor pressure at temperatures of interest for power generation, and is abundant, commercially available in acceptable purity and is relative inexpensive, making it an attractive medium as a reactor coolant, for LMRs, however, it does react violently with water which imposes problems in the design of sodium-to-water steam boilers. In addition, the control of the nuclear process in fast reactors is inherently hard as compared to light water thermal reactors. In order to minimize the risk of a coolant loss due to the rupture of one of the main coolant circulating lines, a pool type reactor is favored over a loop type system.
A schematic of a pool type fast reactor vessel is shown in prior art FIGS. 1 and 1A and described by Hunsbedt et al. (U.S. Pat. No. 5,043,135). FIG. 1 shows one type of a passively cooled liquid nuclear reactor 10 containing a circular reactor vessel 12 containing a pool of liquid metal coolant 14 such as sodium metal or sodium/potassium, for heat transfer, in which is immersed nuclear core 16 containing fissionable fuel. Fission action rate is governed by neutron absorbing control rods, generally shown as 18, moving from or into the nuclear core 16. The reactor vessel 12 is enclosed within a concentric containment vessel 20 in spaced apart relationship all within collector cylinder 22. A silo 24 of, for example concrete, houses the collector cylinder 22 as shown. As shown in FIGS. 1 and 1A, a series of annular downer partitions 28 and riser partitions 30 are formed by this concentric arrangement which partitions form cycling fluid circuit paths. Volume 32 contains cool ambient air 34, which, upon heating, induces a natural convection passing around the bottom of containment collector cylinder 22 and up annular riser partition 30, as shown by the arrows in FIGS. 1 and 1A, to an outlet shown generally as 36. As such, the cooling system is entirely passive and operates continuously by convection and thermal radiation. This prior art, standard design, can be placed in the ground 33.
Conventional nuclear reactors have utilized a variety of elaborate energy driven cooling systems to dissipate heat from the reactor, such as Cachera (U.S. Pat. No. 3,968,653). Liquid metal cooled reactors such as the modular type are disclosed by U.S. Pat. No. 4,508,677, utilizing sodium or sodium-potassium as the coolant.
Passive safety of fast reactors rely on removing “decay heat” through the reactor vessel. Heat continues to be generated by the core even after the fission reactor has stopped. It is important that this decay heat (reactor residual heat/fission product decay heat) can always be removed after reactor shutdown caused by an accident or fault condition. A passively cooled fast reactor system primarily operates continuously through the process of natural convection in fluids, conduction and thermal radiation. Here, decay heat is transported from the heat producing reactor fuel core out to the reactor vessel by means of natural convection flow of the coolant through the primary cooling circuit loop. The transported heat is in turn conducted out through the wall of the reactor vessel and on through the air filled space intermediate the reactor and containment vessel wall is continued on into the surrounding atmosphere, by natural convection to the naturally convecting surrounding air and partially by thermal radiation.
The critical parameter in passive safety is the ratio of the surface area of the reactor vessel outer wall 12 to volume of the core 16 ratio. Since fast reactor cores are very compact, this fact has limited the maximum power that is available while still maintaining a passively cooled core to about 1000 MW (megawatts) thermal. One such type reactor, as described by Hunsbedt (U.S. Pat. No. 5,021,211) undertakes to maintain the bulk of the metal coolant at temperatures below safe limits. Although higher power levels should reduce the capital and operating costs of the fast reactor, high fuel core temperature peaks would be likely to occur if the liquid metal coolant flow through the core is terminated. Thus, removal of decay heat from the fuel core is primarily by heat conduction through an extensive mass of enclosing stainless steel, perhaps eight inches total, which would require a temperature difference between the opposite surface areas of approximately 700° F. (371° C.) to transport the heat from within the core region to an exterior region. Other patents in this area, include, for example, U.S. Pat. Nos. 4,859,402 (Tupper et al.); 5,043,136 (
Referring now to prior art FIG. 2, a general, simplified cross-section of a prior art reactor configuration is shown with a reactor core 116, having a center 117 with liquid metal or salt coolant 114 contacting the fuel and the reactor vessel 120, the inside of the containment vessel 122, with air coolant contained in the annular riser space 130 where entry cool air contacts the outside of the reactor vessel. The radius=1.0 from the core center to the interior of containment collector cylinder 122. Point of cool air contact 134 is shown on cylinder 122.
To reiterate:                1. LMRs have high power density (fast spectrum) and need very effective coolant such as liquid metal or molten salt.        2. If the coolant flow is lost to the LMR, the problem is potentially more severe than for water reactors.        3. Therefore, LMRs require an auxiliary coolant system in case the main one (steam generators) is lost.        4. There are two LMR configurations: loop, which is similar to a PWR and pool which is similar to a BWR.        5. A pool configuration is more effective in removing heat if the coolant flow is lost, because the liquid metal can conduct the heat from the core to the vessel wall where it is taken out by radiation and flow convection.        6. The critical parameter for the pool configuration conduction of heat out is the surface (inner vessel wall) to volume (core) ratio. That is surface area in meters square of inner vessel wall: volume in meters cubed (m3) of the reactor core. The higher the ratio, the more effective the heat removal.        7. Parametric analyses have indicated that 1000 MWt is about the optimum for a pool reactor. Lower powers lack in economics, higher powers give more safety problems;        8. All designs teach concentric volumes from the core outward.        
Thus, there is a need for a new, simpler and revolutionary LMR design, to maximize heat removal efficiencies and be cost effective and commercial. It is a main object to provide a new, innovative simpler core design for LMRs.